Viability of land speeder assassination force?

Just running through some calculations for 11th. Fulguris Task Force gives you T1 deep strike for land speeders (and storm speeders). 8" deep strike means we can get within melta range. I think I remember from one of the preview battles that land speeders will be about 90 points?

Suppose we have 3 land speeders plus a storm speeder hammerstrike (about 400 points total), shooting into an oath target. The storm speeder strips covers and then the land speeders combined have 6 melta shots, 3 lascannon shots, and 24 gatling cannon shots, all with ignores cover, reroll hits and (if codex marines) +1 to wound.

Then add other detachment benefits: e.g., Hammer of Avernii gives reroll 1s to wound against the oath target.

I think this could be a really powerful t1 assassination threat for large targets. That combination of shooting can take out or powerfully reduce all sorts of large targets, like daemon primarchs, big knights, defilers, etc. The deep strike flexibility means they can get to just about wherever they need to be.

The main limitation is that the main enemy units probably won't be in the front line so it could be hard to get them into melta range. We could either stay out of melta range (reducing damage but also staying safer) or wait until t2 and give the enemy something to worry about.

Maybe this is obvious to everyone else already, but this seemed like a really fun and effective combo. Does anyone see downsides that I'm not seeing?